The following abbreviations are herewith defined:    3GPP third generation partnership project    ARQ automatic repeat request    BLER block error ratio    C/I carrier to interference ratio    CQI channel quality indicator    DL downlink    HARQ hybrid ARQ    LTE long term evolution    Node B base station    eNB evolved Node B    OFDMA orthogonal frequency division multiplexing access    OFDM orthogonal frequency division multiplexing    SC-FDMA single carrier-frequency division multiplexing access    PRB physical resource block    PS Packet Scheduler    TTI transmission timing interval    UL uplink    UE user equipment    UTRAN universal terrestrial radio access network    EUTRAN evolved UTRAN    aGW access gateway
A proposed communication system known as evolved UTRAN (E-UTRAN, also referred to as UTRAN-LTE) is at present a study item within the 3GPP. The current working assumption is that the access technique will be OFDMA for the DL and SC-FDMA for the UL, which are both based on OFDM technique and can be expected to provide an opportunity to perform link adaptation and user multiplexing in the frequency domain.
Several publications have reported the results of studies related to frequency domain packet scheduling for OFDM based systems such as UTRAN-LTE. However, these publications do not explicitly discuss how to accommodate the simultaneous scheduling of new data and pending HARQ retransmissions.
As is described in section 9.1.2.5 of 3GPP TR 25.814 V7.0.0 (2006-06), Technical Report, 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Physical layer aspects for evolved Universal Terrestrial Radio Access (UTRA) (Release 7), in general HARQ can be classified as being synchronous or asynchronous.
Synchronous HARQ implies that (re)transmissions for a certain HARQ process are restricted to occur at known time instants. No explicit signaling of the HARQ process number is required as the process number can be derived from, e.g., the subframe number.
Asynchronous HARQ implies that (re)transmission for a certain HARQ process may occur at any time. Explicit signaling of the HARQ process number is therefore required.
In principle, synchronous operation with an arbitrary number of simultaneous active processes at a time instant could be envisioned. In this case, additional signaling may be required. Asynchronous operation already supports an arbitrary number of simultaneous active processes at a time instant. Furthermore, note that, in a synchronous scheme the transmitter may choose not to utilize all possible retransmission instants, e.g., to support pre-emption. This may require additional signaling.
The various forms of HARQ are further classified as adaptive or non-adaptive in terms of transmission attributes, e.g., the resource unit (RU) allocation, modulation and transport block size, and the duration of the retransmission. Control channel requirements can be different for each case.
Adaptive HARQ implies that the transmitter may change some or all of the transmission attributes used in each retransmission, as compared to the initial transmissions (e.g. due to changes in the radio conditions). Hence, the associated control information needs to be transmitted with the retransmission. The changes considered are: modulation, resource unit allocation and duration of transmission.
Non-adaptive HARQ implies that changes, if any, in the transmission attributes for the retransmissions are known to both the transmitter and receiver at the time of the initial transmission. Hence, associated control information need not be transmitted for the retransmission.
With those definitions, the HS-DSCH in WCDMA uses an adaptive, asynchronous HARQ scheme, while the E-DCH in WCDMA uses a synchronous, non-adaptive HARQ scheme.
The capability to adaptively change the packet format (i.e., adaptive IR) and the transmission timing (i.e., asynchronous IR) yields an adaptive, asynchronous IR based HARQ operation. Such a scheme has the potential of optimally allocating the retransmission resources in a time varying channel. For each HARQ retransmission, control information about the packet format needs to be transmitted together with the data sub-packet. This increases overhead associated with retransmission operations
Synchronous HARQ transmission entails operating the system on the basis of a predefined sequence of retransmission packet format and timing.
Non-adaptive HARQ is a good solution to decrease scheduling signaling for re-transmissions. However, for semi-persistent scheduling where initial transmissions are persistently allocated, it is hard to enable non-adaptive HARQ as much as possible. Adaptive HARQ in addition to non-adaptive HARQ is essential for semi-persistent scheduling because some resources for retransmissions are already occupied by initial transmissions of other users through persistent allocation.
The preliminary results show the proportion of non-adaptive HARQ is only about 30-40% at capacity point of semi-persistent scheduling (when using non-adaptive HARQ as much as possible). The remaining 60-70% of retransmissions are using adaptive HARQ (a ‘Grant’ along with NAK is sent), and then large signaling for retransmission is needed, which is the main parts of control signaling for semi-persistent scheduling.
Accordingly, those skilled in the art seek methods, apparatus and computer program products that would enable the increased use of non-adaptive HARQ to decrease control signaling.